because it is harder. most board type sports all have a similar definition for unatural. like frontside and backside for boarders, or heelside or toeside for skaters.
its the same concept as forward or regular, and backwards or switch. switch is certainly harder then riding forwards. it just helps us skiers define tricks and levels of difficulty for comps. and what not.
besides it makes you a better all around rider being able to carve both directions and throw tricks. I have always pushed myself to learn both ways with everything. it took a long time but its fun.
basically its simple: switchin-dirty- means I'm going switch and spinning to my right, and its HARD AS FUUUUUUUUCK! but I can spin both ways forward with 1's and 3's solid all day long off anything. but all I can do switch-rightside are 180's decently. I don't even like buttering and turning around that way, even though I do I think about it alot more. but I am always pushing to get more tricks on that side. becuase if I can master that then theres nothing I can't do in my mind. so it is important because it matters to oneself, others(in my case my friends who I ride with), and judges if your competing. I enjoy watching riders with a variety of tricks both ways specially off natural terrain, uuhh snowboarders in particular, but maybe you just don't care if a rider is more complete.